Fuel bundles in boiling water nuclear reactors require spacers for the maintenance of their fuel rods in designed spaced apart relation. Such fuel bundles have an array of upstanding side-by-side fuel rods supported at the bottom on a lower tie plate. At least some of the fuel rods extend to an upper tie plate. All fuel rods are surrounded between the tie plates by a fuel bundle channel. The lower tie plate is configured to permit the inflow of moderating water coolant while the upper tie plate permits the outflow of both water coolant and generated steam. The surrounding channel confines the flow of coolant to a path around the steam generating fuel rods separate from a water flooded core bypass region surrounding each fuel bundle.
The generation of steam within a boiling water reactor fuel bundle can be simply understood. At the bottom of the fuel bundle, liquid coolant is pumped into, enters and rises upwardly within the channel and around the fuel rods. As the coolant proceeds upwardly, steam is generated in an increasing fraction within the upward coolant flow. As a result, the upper portion of the fuel bundle is typically referred to as the "two phase" region of the fuel bundle, these two phases being water and steam. An increasing amount of steam or "void fraction" is generated as the coolant rises and passes out of the fuel bundle through the upper tie plate.
The flow in the upper two phase region of the fuel bundle is characterized by differential flow rates between the upwardly flowing water and the upwardly flowing steam. The upwardly flowing water tends to adhere to and cover all available surfaces. This upward flow of water usually occurs at a relatively low velocity when compared to the upward flow rate of the generated steam. The upwardly flowing steam tends to move to all open spaces within the fuel bundle and away from all surfaces. This upward flow of steam occurs at a relatively high velocity when compared to the upward flow rate of the water.
It is common to compare the upward flow rate of steam in a fuel bundle geometry to the upward flow rate of water in a ratio known as the "slip ratio." This slip ratio always constitutes a number greater than one and usually falls with the range of 2 to 20.
In a regular row and column array of fuel rods, the greatest volume of upward steam flow occurs in the so-called "subchannel" volume. This region constitutes a roughly cylindrical volume between four adjacent fuel rods. The steam-water mixture in the subchannel is mostly steam flowing upwardly at a relatively high velocity while the surrounding volumes adjacent to the fuel rods have a higher concentration of slower moving water.
The fuel rods extending between the tie plates are long slender fuel pellet filled sealed tubes. Absent any restraint during the fluid flow and steam generating process, these fuel rods would vibrate from their original designed spacing and most likely come into abrading contact. To prevent this abrading contact as well as to maintain the fuel rods in their original design spacing for nuclear efficiency, it is the regular practice of the nuclear industry to place fuel rod spacers interior of the fuel bundles.
Typically such fuel rod spacers individually surround and hold each fuel rod at the particular elevation of the spacer. These spacers are placed at sufficient selected vertical intervals to minimize fuel rod vibrations and to prevent abrading contact and maintain designed fuel rod spacing.
Fuel bundle spacers of various varieties have been developed. Such spacers include cell type spacers made of spring steel (commonly Inconel) in which a small matrix of spring steel surrounds each of the fuel rods, ferrule spacers in which each fuel rod is surrounded by its own individual ferrule, and egg crate spacers in which a generally square grid defines discrete cells surrounding each fuel rod.
When the generation of steam within a fuel bundle is considered, egg crate spacers have an inherent disadvantage. The points of connection of the grid are in the subchannel volumes between the fuel rods. This intersection of the grid is in the middle of the high velocity upward flowing steam. With this intersection, upward steam flow is inhibited.
This problem has been recognized in the prior art. Specifically, in prior art spacer construction it has been suggested to eliminate the grid intersections and substitute vertically upstanding subchannel tubes at the intersections of the grid. This spacer type has been called a "cross-point" spacer. The present invention is directed to spacers of the cross point type. Instead of having the grid secured by an intersection interfering with high velocity steam flow in the subchannel volume, the grid is secured by the outside of the subchannel tube. Thus, the exterior of the subchannel tube provides the necessary grid interconnection. Further, the interior of the subchannel tube provides the necessary subchannel volume for upward steam flow. This has the advantage of taking the intersection of the grid out of the subchannel volume and providing the cylindrical volume required for the subchannel flow interior of the tube for the upward steam flow.